Charge preamplifier

ABSTRACT

The charge preamplifier comprises an input stage driven by the signal to be amplified, the input stage being connected to an amplification stage associated with an output stage, and a circuit for applying optoelectronic feedback from the output to the input of the preamplifier. The input stage comprises a cooled field-effect transistor, the signal to be amplified being applied to the input gate of the transistor. The load resistance of the transistor is provided by a second field-effect transistor mounted in series with an inductance coil.

This invention relates to a charge preamplifier.

It is known that semiconductor detectors of the lithium-compensatedsilicon type are advantageously employed in the fluorescence analysis oflight elements and therefore in the detection of low-energy X-rays. Thedetector is associated with a measuring system which is intended tomeasure the number of counts as a function of energy, tht is to say thenumber of pulses delivered by said detector, each pulse being intendedto correspond to the arrival of a particle. The measuring system mustclearly have a very high resolving power which in fact depends on thequality and dimensions of the detector as well as the associatedelectronic circuit and in particular on the charge preamplifier which isplaced at the head of this electronic circuit. In general, efforts haveconsequently been directed to the reduction of noise of the componentswhich constitute the input stage of the preamplifier. In order to reducethis noise, a certain number of devices have already been constructed.

The prior art can be illustrated by the preamplification devicedescribed in the review entitled "Nuclear Instrument and Methods," 71,1969, pages 273 to 279. In this device, the input of the preamplifier isconstituted by a field-effect transistor which will be designatedhereinafter as a FET, said transistor being placed within a cryostat inthe vicinity of the detector, thus reducing its noise as well as straycapacitances. Moreover, a new feedback technique in the so-calledoptoelectronic field has been adopted for the preamplifier and permits avery appreciable improvement in the resolving power irrespective of thecount rate. The feedback resistor is replaced by an electroluminescentdiode which is intended to shine on the input FET. The noisecontribution from the feedback element is much lower in this case.However, further high noise sources also exist and are primarily due tothe nature of the resistance of the load on the input FET.

A clearer understanding of this problem will be gained by referring tothe accompanying FIG. 1 in which the input stage of a preamplifier inaccordance with the prior art is shown diagrammatically.

In this figure, the signal delivered by the detector is applied to theinput gate G of the field-effect transistor T' which is cooled. Thesource input of the transistor is connected to ground whilst its draininput is connected to the emitter of the bipolar transistor T. Thecommon point of the two transistors is connected to the supply line 2through the emitter resistor R₁. The collector of the transistor T isconnected to the second supply line through the collector resistor R₂.The output of the input stage of the preamplifier is located at point A.

The noise sources which disturb the measurements have different origins.These noises are given by the following formulae:

i₀ ² represents the intensity of the noise produced by the channelcurrent of the transistor T',

i_(b) ² = 2qI_(b) represents the partition noise produced within thetransistor T.

The resistors R₁ and R₂ produce not-negligible levels of noise,

i₁ ² = (4KT/R₁) represents the intensity of the noise produced by theresistor R₁.

i₂ ² = (4KT/R₂) represents the intensity of the noise produced by theresistor R₂.

In these formulae, q is the elementary charge, K is the Boltzmanconstant and T is the absolute ambient temperature.

It is noted on the one hand that the values of the resistors R₁ and R₂must be high in order to reduce the levels of noise i₁ ² and i₂ ² and onthe other hand that the transistor T must be selected from those whichhave a very high current gain in order to reduce the noise i_(b) ². Thefirst of these observations presupposes a high value of the inputvoltage which is not always possible in practice. Moreover, it isrecommended practice to employ a FET instead of the bipolar transistor Twhen the drive is applied through a high resistance as in this instance.The n-type transistor could be replaced only by a p-type FET.Unfortunately, the noise level of this latter is much too high comparedwith an n-type transistor and even with a bipolar transistor.

This invention is precisely directed to a charge preamplifier whichovercomes th disadvantages mentioned in the foregoing by suppressing orat least considerably reducing the noise produced by the input stage ofthe preamplifier.

The charge preamplifier essentially comprises an input stage driven bythe signal to be amplified, said stage being connected to anamplification stage which is in turn associated with an output stage,and a circuit for applying feedback from the output to the input of saidpreamplifier, said feedback being of the optoelectronic type, said inputstage being such as to comprise a cooled field-effect transistor, thesignal to be amplified being applied to the input gate of saidtransistor, the load resistance of said transistor being provided by asecond field-effect transistor mounted in series with an inductancecoil.

In accordance with a first characteristic feature of the invention, theimpedance of said inductance coil within the frequency range of thepreamplifier is considerably higher than the reciprocal of the slope ofthe field-effect load transistor.

In accordance with a second characteristic feature, the drain output ofthe input transistor is connected on the one hand to the source input ofthe load transistor through said inductance coil which is mounted inseries with a resistor and on the other hand to the input gate of saidload transistor.

In accordance with a third characteristic feature, the input stage isfollowed by a differential stage comprising a field-effect transistorand a bipolar transistor.

In accordance with a further characteristic feature, the feedbackcircuit comprises a hysteresis comparator having one input connected tothe output of the preamplifier output stage and the other outputconnected to a reference voltage source, the output of said comparatorbeing connected to a device for initiating the supply of anelectroluminescent diode placed opposite to the input transistor througha capacitor in which one plate is connected to the output of thepreamplifier output stage and the other plate is connected to the inputgate of the input field-effect transistor.

A more complete understanding of the invention will in any case beobtained from the following description of one embodiment which is givenby way of example but not in any limiting sense, reference being made tothe accompanying drawings, in which:

FIG. 1 as hereinabove described is a diagram of the input stage of apreamplifier in accordance with the prior art;

FIG. 2 is a diagram of the input stage of the preamplifier in accordancewith the invention;

FIG. 3 is a circuit diagram of the complete preamplifier in accordancewith the invention;

FIG. 4 shows two curves which give the resolution as a function of thecount rate in the case of the preamplfier according to the invention andin the case of a preamplifier of known type.

The input of the preamplifier in accordance with the invention is shownin detail in FIG. 2. There can be seen in this figure the FET T'₁, theinput gate of which is driven by the signal delivered by the detector.The source input of the FET T'₁ is connected to ground whilst the drainof said FET is connected on the one hand to the source input of the loadFET T'₂ through the inductance coil L₁ and on the other hand to the gateof the FET T'₂. The drain of the FET'₂ is connected to the line 4 forsupplying voltage at +12 volts. The drain of the FET T'₁ (at the pointA) is connected to the gate of the FET T'₃ which constitutes adifferential circuit in conjunction with the bipolar transistor T₁. Thesource of the FET T'₃ and the emitter of the transistor T₁ are connectedto the -12 volt supply line 4 through the bias resistor R₂. The valuesof the supply voltages are obviously given only by way of example andcorrespond to types of components which are the most widely employed.The bipolar transistor T₂ constitutes a parallel cascode circuit inconjunction with the transistor T₁. The collector of the transistor T₁and the emitter of the transistor T₂ are connected to the line 2 throughthe resistor R₇. The three transistors T'₃, T₁ and T₂ constitute thesecond amplification stage of the preamplifier. The collector output(point B) of the transistor T₂ is connected to the output stage of thepreamplifier.

In this circuit arrangement, the transistor T'₁ is provided with a loadresistance by the field-effect transistor T'₂. Gain degeneration of theFET T'₂ is caused by the presence of the inductance coil L₁ in order toreduce as far as possible the variations of the current which passesthrough said FET. The noise intensity i² at the point A' is thenobtained substantially as follows: ##EQU1## wherein i_(D1) ² and i_(D2)² represent respectively the noise produced by the channel current ofthe FETs T'₁ and T'₂. In this formula, S₂ is the slope of the FET T'₂.It is clearly necessary to ensure that the impedance L₁ ω issubstantially higher than the term 1/S₂ within the frequency rangeemployed by the filter of the amplifier. The value adopted for theinductance coil L₁ is approximately 200 to 300 mH, this value beingdetermined experimentally by establishing that the noise level measuredby the quadratic voltmeter at the output of the amplification circuitbecomes practically insensitive to the type of transistor which ischosen as a load device. In the case of the transistor T'₂, it ispreferable to employ a transistor of type 2N 4416 (Texas) and the sameapplies to the transistor T'₁.

The FET T'₃ is selected from those which have a high slope although thevalue of current I_(D) SS must nevertheless not be too high, I_(D) SSbeing the value of the drain current in respect of a zero gate-sourcebias. The transistor T₁ which is the second element of the differentialstage is preferably a bipolar transistor having a current gain ω whichis at least equal to 200. In this circuit arrangement, there remainsonly one noise generator represented by the noise intensity of thetransistor T'₁ since the noise of the transistor T'₂ can be consideredas negligible. The Miller effect produced by this arrangement haspractically no effect on the signal-to-noise ratio of the amplifier.

There is shown in FIG. 3 one example of construction of the preamplifierin accordance with the invention and comprising the input stage whichhas already been described with reference to FIG. 2. This figure showsthe detector 6 (which is for example of the lithium-compensated silicontype) supplied by a voltage source -HT through a filter if necessary.The detector 6 and the FET T'₁ are placed within a cryostat as shown bythe dashed outline 7 and maintained at a very low temperature. The loadresistance of the transistor T'₁ comprises in addition to the transistorT'₂ and the inductance coil L₁ a resistor R₁ which is mounted in serieswith said inductance coil.

The amplification stage proper of the preamplifier further comprises thebipolar transistors T₁ and T₂, a filter network 8 and a load transistorT₃. The filter network 8 comprises in known manner the resistors R₈, R₉,R₁₀ and the capacitor C₆. The filter circuit 8 is connected to thetransistor T₂ through the diode D₃ and to the collector of the loadtransistor T₃ through the diode D₁. The filter circuit 8 which is alow-pass filter is intended to correct the frequency response of thepreamplifier. There are also shown in the figure the differenttransistor bias resistors, namely the resistors R₆ and R₃ for thetransistor T₃, the resistor R₄, the resistor R₅ and the potentiometer P₁for the transistor T₁, the resistor R₅ for the base of the transistorT₂. Moreover, the supply lines 2 and 4 for the amplification stage andthe input stage are connected to the voltage sources through twocoupling cells 10 and 12 respectively which are constituted respectivelyby the resistor R₂₆ and the capacitor C₄, and by the resistor R₂₅ andthe capacitor C₅.

The output stage 14 of the preamplifier is essentially constituted bythe bipolar transistors T₄ and T₅, the bases of which are connectedrespectively to the collectors of the transistors T₂ and T₃. There arealso shown the protective resistors R₁₁ and R₁₄ of these transistors andthe emitter resistors R₁₂ and R₁₃ in which the point of connectionconstitutes at the same time the output S of the preamplifier.

The preamplifier further comprises a feedback system of theoptoelectronic type. This system comprises a bipolar transistor T₆, thebase of which is connected to the point of output S of the preamplifierand the emitter of which is connected to the reverse input of ahysteresis comparator M₁ through a voltage divider which also serves tobias said transistor T₆ and is constituted by the resistors R₁₆ and R₁₅.The forward input of the comparator is connected to a reference voltagesource constituted by the potentiometer P₃ which is supplied between thevoltages of -5 volt, +5 volts, and by the resistor R₁₇. The comparatorM₁ comprises a feedback circuit constituted by the resistor R₁₉ and thepotentiometer P₂. The potentiometer P₃ makes it possible to adjust thecentering of the comparator M₁ whilst the potentiometer P₂ serves toadjust the dynamic output characteristics of the preamplifier. The(logical) output of the comparator M₁ is connected to the input of thelogical gate M₂. The second input of the logical gate M₂ is alsoconnected to the output of the comparator M₁ through an integratingcircuit 16 constituted in known manner by the inductance coil L₂ and bythe resistor R₂₂. This integrating circuit serves to introduce atime-delay in the logical signal delivered by the comparator M₁. Theoutput of the logical gate M₂ which is advantageously constituted by aNOR gate drives a circuit 18 for controlling an electroluminescent diodeD₄ placed within the cryostat 7 opposite to the FET T'₁. The emitteroutput of the transistor T₉ is preferably connected to the supply line 4through the resistor R₂₀. The transistor collector T₉ comprises theresistor R₂₁ and the base of the transistor T₈ is connected to a voltagesupply of +5 volts. The transistor T₇ which is associated with thetransistor T₈ constitutes the bistable device 20.

The output S of the preamplifier is also connected to a feedbackcapacitor C₈ placed between the detector 8 and the FET T'₁. There isalso placed within the cryostat 7 a Zener diode Z₁, one terminal ofwhich is connected to a voltage supply of +12 volts through the resistorR₂₈.

The operation of the preamplifier is as follows. When the action of thecharges stored in the detector is such that the voltage delivered by thepreamplifier at its output S is higher than the threshold value presetby means of the potentiometers P₂ and P₃, the hysteresis comparatorcircuit M₁ delivers the logical signal O. At the end of the period tcorresponding to the time-delay introduced by the integrating circuit16, two signals of logical level O appear at the inputs of the logicalgate M₂ which is constituted by a NOR circuit. Said gate thereforedelivers a signal of logical level 1 which initiates the transition ofthe circuit 20. This reversal of state triggers the transistor T₉ intoconduction and this latter therefore supplies the electroluminescentdiode T₄. Said diode shines on the gate-drain junction of the FET T'₁.The resultant increase in the reverse current within said junctioncauses the feedback capacitor C₈ to discharge for a period of time whichdepends on the current within the transistor T₉ and therefore on thevalue of the resistor R₂₀. When the output voltage of the preamplifierattains the bottom threshold value determined by the hysteresiscomparator M₁, the bistable circuit 20 returns to its initial state,with the result that the current within the diode D₄ is cut-off. Theoutput I of the comparator M₁ constitutes an inhibition output for thedevice which serves to record the output voltage S of the preamplifier.

The drain voltage of the FET T₁ can be adjusted by means of thepotentiometer P₁ to a value of the order of 5 volts. Depending on thetransistor which is employed, it is usually necessary to set this valuebetween 4 and 5 volts so as to obtain the best resolution. The resistorR₁ makes it possible to control the drain current of the transistor T'₁independently of its drain voltage. A value of 500 ohms is preferablyemployed for the resistor R₁ in the case in which the input FETs are ofthe type designated as 2N 4416 (Texas). The potentiometers P₂ and P₃make it possible to adjust the dynamic output characteristics within arange which, in the example described, has a maximum value of +1.5 voltsand a minimum value of -1.5 volts.

Under these conditions, in the case of count rates of 4300 counts persecond and 130,000 counts per second, the reset period is respectivelyequal to 186 ms and 4.4 ms in the case of a Fe-55 source (of the orderof 6 keV energy). The time of return of the preamplifier is adjustableby means of the resistor R₂₀ and is usually set at a value below 10 μs.

The temperature to which the transistor T'₁ is usually brought is in thevicinity of -120°C in order to minimize its noise level. The optimumvalue is obtained by regulating the current flowing through the Zenerdiode Z₁ which is placed in the vicinity of the transistor T'₁. To thisend, the value of the resistor R₂₈ is determined experimentally by meansof a quadratic voltmeter placed at the output of the amplifier. By wayof example in the case of the assembly consisting of cryostat andpreamplifier which is employed in the inventors' laboratory, the optimumvalue of the resistor R₂₈ was 62 ohms.

In FIg. 4, the resolution of the preamplifier (expressed in eV) isrepresented as a function of the count rate (expressed in counts persecond on the logarithmic scale) in the case of a Fe-55 source. Thecurve H₁ corresponds to a preamplifier exactly as described above andemployed in the prior art. The curve H₂ corresponds to the preamplifierin accordance with the invention. This figure shows that in respect of agiven resolution, the count rate is approximately twice as high with thepreamplifier in accordance with the invention.

The foregoing decription corresponds to the operation of thepreamplifier in a pulsed regime. However, the operation of someamplifiers may be disturbed by a preamplifier which operates in a pulsedregime. In that case it is possible to modify the preamplifier to aslight extent so that the transistor T'₁ should be continuouslyilluminated by the electroluminescent diode D₄. The modificationconsists in connecting the base of the transistor T₇ to the analogoutput S of the preamplifier instead of joining it to the output of thegate M₁. The base of the transistor T₈ is then connected to ground, withthe result that the transistors T₇ and T₈ constitute a Class-Adifferential stage.

What I claim is:
 1. A charge preamplifier essentially comprising aninput stage driven by the signal to be amplified, said stage beingconnected to an amplification stage which is in turn associated with anoutput stage, and a circuit for applying feedback from the output to theinput of said preamplifier, said feedback being of the optoelectronictype, said input stage being such as to comprise a field-effecttransistor cooled below ambient temperature, the signal to be amplifiedbeing applied to the input gate of said transistor, the load resistanceof said transistor being provided by a second field-effect transistormounted in series with an inductance coil.
 2. A preamplifier accordingto claim 1, wherein the impedance of said inductance coil within thefrequency range of the preamplifier is considerably higher than thereciprocal of the slope of the field-effect load transistor.
 3. Apreamplifier according to claim 1, wherein the drain output of the inputtransistor is connected on the one hand to the source input of the loadtransistor through said inductance coil which is mounted in series witha resistor and on the other hand to the input gate of said loadtransistor.
 4. A charge preamplifier essentially comprising an inputstage driven by the signal to be amplified, an amplification stageconncted to said input stage, an output stage associated with saidamplification stage, and a circuit for applying feedback from the outputto the input of said preamplifier, said feedback being of theoptoelectronic type, said input stage comprising a first field-effecttransistor cooled below ambient temperature, the signal to be amplifiedbeing applied to the input gate of said first field-effect transistor, asecond field-effect transistor connected in series with an inductancecoil as the load resistance of said first field-effect transistor, saidfeedback circuit comprising a comparator having one input connected tothe output of said output stage and the other input connected to areference voltage source, the output of said comparator being connectedto means for initiating the supply of an electroluminescent diode placedopposite to the input of said first field-effect transistor, and acapacitor having one plate connected to the output of said output stageand the other plate connected to the input gate of said firstfield-effect transistor.
 5. A preamplifier according to claim 4, whereinthe feedback circuit comprises a delay circuit at the output of thecomparator, a bistable circuit controlled by the logical state of theoutput of the comparator, said circuit being capable of supplying abipolar transistor whose collector is connected to the input of theelectroluminescent diode.